(1) Field of the invention
The present invention relates to an intake system with double intake ports for internal combustion engine, especially, which is made obtainable high volumetric efficiency and coefficient of air utilization through broad speed range.
(2) Description of prior arts
Generally, a so-called intake system with double intake ports for internal combustion engine, which is provided with two intake ports for each cylinder chamber, is adopted to increase the volume of suction air. Usually, according to such intake system, as seen in FIG. 1, the two intake ports 2, 3 for a cylinder chamber 1 of an internal engine E are formed in the cylinder head 6 and located side by side in the parallel direction A with the axis S of the cam shaft 5 of a valve mechanism 4. The inlets 7, 8 of these intake ports 2, 3 are opened at the same lateral side 10 of the cylinder head 6, and the outlets 11, 12 thereof at the top end 14 of the cylinder chamber 1.
Among such intake systems which is provided with fundamental structure above mentioned, there is one, as described in Japanese official patent No. 52-7485 or U.S. Pat. No. 4,523,560 for example, having an intake port consisted of a directional port and another intake port consisted of a helical port for each cylinder chamber. The directional port is made to introduce suction air without helical flow, and the helical port is made to introduce suction air with helical flow, into the cylinder chamber. In these prior arts, the outlet of the directional port is located farther than the center of cylinder chamber from these inlets, and that of the helical port is located nearer than the center of cylinder chamber to these inlets. And the helical port is bent like a crank in order to arrange the length of these ports about equal.
According to such intake system with double intake ports which are largely varied in the distance from the axis of the cam shaft to each outlet, if the suction valves were arranged to be driven by a common cam shaft, a bridge member and a guide means for guiding its linear action are usually involved in the valve mechanism to harmonize the timings, the lifts and the power for opening of both valves. Therefore, the valve mechanism is made complex and bulky. Moreover, as the helical port is bent like a crank, the resistance against suction air is so large that the swirl and dynamic effects of suction air are weakened to reduce volumetric efficiency. Furthermore, manufacturing of the helical port is difficult for its complex figure. In addition to above, to say nothing of large engine, it is difficult to obtain preferable swirl when usual intake system with double intake ports was adopted to a small diesel engine which requires powerful swirl with many eddies.
By the way, there is such intake system, which has the fundamental structure above mentioned, wherein only one of the two intake ports is used for suction in lower speed range in order to increase volumetric efficiency by speedup of suction flow. According to this intake system, the primary intake port, which is used through whole speed range, is a helical port, as described in Japanese official open patent No. 58-53632 (cf. page 6, from the last line of lower left column to the eighth line of lower right column), for example, in order to produce effective swirl for improvement of volumetric efficiency even in slow speed range and light load condition.
According to this prior art, in higher speed range, the amount of suction air is increased by using two intake ports, and, in this consequence, the volumetric efficiency is increased. However, in higher speed range, as another one of the two intake ports, a directional port, is made to introduce suction air with straightway flow into the cylinder chamber, to say nothing of large engine, the swirl formed in the cylinder chamber is not sufficiently powerful to ensure good combustion in small, high power and high speed engine which especially requires such powerful swirl with many eddies. And, in the lower speed range, the resistance against suction air is largely increased by shutoff of the directional port, so that the volumetric efficiency and coefficient of air utilization drop.
On one hand, among such intake systems, which are provided with the fundamental structure, and which are made to use only one of two intake ports for raise of suction air speed and volumetric efficiency in lower speed range, as seen in Japanese official utility model No. 59-8246, there is one which involves two intake manifolds, having divided passage communicated to corresponding intake port, and a pause valve for closing one of the two divided passages, which valve is shut off in slower speed range.
However, in this prior art, through whole speed range of the engine, both suction valves are active, so that the part of the one divided passage and the intake port following to the pause valve are communicated with the cylinder chamber. Therefore, the back pressure in the cylinder chamber at the suction stroke and the volumetric efficiency are decreased, and the suction air introduced into the cylinder chamber through the other intake port is blown into the pausing intake port to reduce coefficient of air utilization. Thus, the power or torque of the engine cannot be made higher. Moreover, the back pressure is so much decreased to reduce speed of suction air, that the swirl is weakened, and that coefficient of mixing of fuel and air is reduced. Furthermore, it is difficult to shut off the divided passage by the pause valve entirely, and leakage of suction air from cylinder chamber through pausing intake port makes swirl weaker.
On the other hand, among such intake systems, which are provided with the fundamental structure, and which are made to use only one of two intake ports for raise of suction air speed and volumetric efficiency in lower speed range, there is one which involves one of the two suction valves being controlled to pause under slow and small load condition, and an orifice located near the outlets of both intake ports, communicating the pausing intake port to the acting intake port so as to enforce the helical flow of the suction air through the latter, or to produce a helical flow to the suction air introduced through the latter into the cylinder chamber, with air passing through the orifice, as seen in Japanese official open utility model No. 59-179240.
However, according to this prior art, the intake ports are communicated with one another in their way, so that the speed of suction flow is not increased and that it is hard to make swirl more powerful, and volumetric efficiency higher, sufficiently.
Beside them, among such intake systems, which are provided with the fundamental structure and which are made to use only one of two intake ports in lower speed range for raise of suction air speed and volumetric efficiency, and which have two divided intake passages communicated to individual intake ports respectively, and a pause valve which closes one of the two divided intake passages in slower speed range, there is one which further involves a resonance pipe (or air chamber of large volume), to which the two individual intake ports are communicated parallelly and respectively through divided intake passages, as seen in Japanese official open utility model No. 59-152138, for example.
According to this prior art, the speed N of the engine which is harmonious with the natural frequency of pulsation of suction air is concluded as following formula (1), wherein L is the length of intake passage; A is the sectional area of intake passage; V is the volume of the part following to the air chamber of large volume in the intake passage; K is a constant: ##EQU1##
In these parameters, sectional area of intake passage A and the volume V are varied by shutoff of the pause valve. But the length of intake passage L is not variable, so that, it is difficult to vary natural frequency of pulsation of suction air for harmonizing with the speed of the engine through broad speed range. And in a certain speed range, the super-charging by inertia effect is not obtained so sufficiently that the volumetric efficiency cannot be made higher.